perf: Use var for METRICS

Each perf module exposes it to the scripting module to report metrics
after all tests have been run. The symbol's scope should allow that,
so declare it as var.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/440

js/perf/core.js

@@ -10,7 +10,7 @@ const Scripting = imports.ui.scripting;
 // someone should be able to get an idea of how well the shell is performing
 // on a particular system.
 
-let METRICS = {
+var METRICS = {
     overviewLatencyFirst:
     { description: "Time to first frame after triggering overview, first time",
       units: "us" },

js/perf/hwtest.js

@@ -2,7 +2,7 @@ const { Clutter, Gio, Gtk, Shell } = imports.gi;
 const Main = imports.ui.main;
 const Scripting = imports.ui.scripting;
 
-let METRICS = {
+var METRICS = {
     timeToDesktop:
     { description: "Time from starting graphical.target to desktop showing",
       units: "us" },